every.channel: sanitized baseline

evolution/proposals/ECP-0010-time-synchronized-chunking.md

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+# ECP-0010: time-synchronized chunking
+
+Status: Draft
+
+## Problem
+
+To achieve byte-identical chunks across uncoordinated antennas, we need a shared timeline for chunk boundaries. If chunk boundaries are derived from local capture start time, identical broadcasts will diverge.
+
+## Decision
+
+Define a **time-synchronized chunking model** based on broadcast clocks:
+
+1. **Primary clock: PCR**
+   - Use MPEG-TS PCR (27 MHz) as the canonical timeline for chunk boundaries.
+2. **UTC anchor (when available)**
+   - ATSC: use PSIP STT to compute UTC time.
+   - DVB: use TDT/TOT to compute UTC time.
+3. **Chunk boundary rule**
+   - Compute `chunk_index = floor((t_anchor) / D)` where `t_anchor` is UTC time when available, otherwise PCR time.
+   - `D` is fixed chunk duration (e.g., 2000 ms).
+4. **Alignment policy**
+   - On ingest, drop data until the next full boundary.
+   - Each chunk contains frames/packets whose PTS fall within `[chunk_start, chunk_end)`.
+5. **Discontinuity handling**
+   - If PCR/PTS discontinuity is detected, close the current group and start a new one at the next boundary.
+6. **Sync status**
+   - Streams with UTC anchor are marked `synced` and eligible for swarm convergence.
+   - Streams without UTC anchor are `unsynced` and remain source-scoped.
+
+## Consequences
+
+- Identical broadcasts on different antennas converge to the same chunk boundaries and can produce byte-identical object streams.
+- Early ingestion can begin unsynced and promote to synced when broadcast time appears.
+- Chunk IDs become deterministic functions of (UTC or PCR timeline, duration).
+
+## Alternatives considered
+
+- Local wall clock (NTP/PTP): rejected as primary anchor because it is not broadcast-authoritative and varies between hosts.
+- Capture-start-based chunking: rejected because it prevents convergence without coordination.
+
+## Rollout / teardown
+
+- Add a time model to the ingest pipeline (PCR + optional UTC anchor).
+- Require chunkers to align to the computed boundary.
+- Promote `StreamId` from source scope to broadcast scope only when `synced`.
+- Supersede this ECP if MoQ introduces a standardized global timeline.